What is the difference between a “bolt” and a “screw”? Surprisingly, many in our industry do not know the difference, or have never really thought about it. To begin with I will give you the definitions that the IFI (Industrial Fasteners Institute) gives in their Fasteners Standards handbook.
BOLT: A bolt is a headed and externally threaded mechanical device designed for insertion through holes in assembled parts to mate with a nut and is normally intended to be tightened or released by turning a nut.
SCREW: A screw is a headed and externally threaded mechanical device possessing capabilities which permit it to be inserted into holes in assembled parts, of mating with a pre-formed internal thread or forming it’s own thread, and of being tightened or released by torquing its head.
An example of a bolt would be a hex head or pan head fastener designed to bolt together two plates with holes drilled in them. You would stack the plates on top of each other - lining up the holes - and inset the bolt through the holes. You would then spin a nut on the threads and tighten the fastener by turning the nut with a wrench while holding the bolt stationary with another wrench or a driver in the case of a recessed part. Many of the hex head and double-hex head fasteners in our industry are designed as bolts but there are some fasteners with a driver recess that are designated as bolts since they were designed to be installed in this manner.
An example of a screw would be a fastener designed to hold the sheet aluminum skin on an airframe. The underlying structural framework would be tapped with threads. You would place the sheet of aluminum with holes predrilled in the corresponding locations over the framework and tighten the screw by turning the fastener itself with a driver. Where it gets confusing is that there are many hex head screws, which look like what you would consider to be a bolt, that are designed to be installed by turning the fastener itself. This makes the part a screw. Another good example of a screw would be a self-tapping fastener that makes it own threads when installed – like a sheet metal or a wood screw.
I find it easiest to remember this by thinking of a bolt as any fastener that you hold stationary while you turn the nut and a screw is any fastener that you turn the entire fastener with a driver or a wrench.
There are certainly some exceptions to these definitions and in some cases bolts may be used as screws and screws may be used as bolts. The part descriptions (especially in the case of the NAS or MS standard parts) usually state whether the component is designed as bolt or a screw. This gives the user an idea of what the fastener was originally designed to do.
Some design features may also differ between a bolt and a screw. Many commercial grade hex head bolts do not have a stepped round bearing surface (or ‘washer pad’) under the head. Since the bolt is designed to be stationary when installed this is not really an issue. However, if this type of bolt were used as a screw, there would be no bearing surface to rotate against the installation surface as the part is tightened and the hex form could then ‘dig’ into the surface, damaging it. This is why all protruding hex head type screws have this round bearing surface. Usually a screw in this configuration is designated as a ‘hex head cap screw’.
Questions, comments?
E-mail me at: ed.spanknoble@heartlandfasteners.com
Monday, March 16, 2009
Tuesday, January 13, 2009
Fatigue
It is a sure bet that most of us have felt the effects of fatigue at one time or another. If I had to explain why I felt fatigued, I would say that the pressures and stresses of my job were bearing down upon me over a sustained period of time. Some days the stress was heavy and some days not so much but the continued cycling of the stresses combines, over time, to make me feel fatigued.
Many of the parts that we put into airplanes are designed with that situation in mind. There are two types of load stresses acting on aircraft structures; static load and dynamic load. Static strength is the ability of a part to resist a very high applied load only one time. Dynamic strength is the ability of that part to resist an applied fluctuating load of lower intensity many times.
Some of you may have flown on an airliner and noticed the wings fluctuating up and down slightly as the plane flew along. The wings have to be able to yield to the many forces that influence them. Each one of these fluctuations creates a dynamic load on the parts in the wing and it is the challenge of engineers to decide what forces the wing will see, how much the wing must yield to the changing force applications, and how many times the collection of parts in the wing will survive the dynamic load before they become “fatigued”.
In order to determine this, we “fatigue test” parts. A fatigue test is basically a long term tension test using lower loads than the part could sustain in a static environment. For fasteners, we put the bolt in the fatigue testing machine and grab on to the head and the thread of the part and pull on it with predetermined loads. We set the machine up to apply a “high load” which is usually somewhere around 37% to 45% of the ultimate load the fastener is expected to bear under static conditions. We then alternate that “high load” to a “low load” which is typically 10% of the “high load”. One application of both the high and low load is called a “cycle” and the machine applies these cycles very fast. Look at this in terms of a stressful job that might have you so fatigued – you have bad day on Monday and a somewhat stressful day on Tuesday followed by another bad day on Wednesday. If this “cycle” continues then there will likely be a failure.
We can easily illustrate fatigue by using something that is probably littering your desk right now. Grab a paperclip. You can bend it by hand. It was designed to be somewhat flexible. It expands when you slide it over the papers and then contracts to grip the papers – like the wing of the airliner we talked about earlier. You can imagine a tool that could apply enough load on the paperclip (ultimate static strength) to snap it in half but you cannot do it with your fingers. Grab one end of the paperclip and bend it out. You have just applied a dynamic load. If you keep bending the wire it will get easier and easier to bend until the wire breaks. You have just fatigued the metal to the failure point using far less load than it would have taken to cut the wire.
Questions or comments? Post them below or email me at: ed.spanknoble@heartlandfasteners.com
Many of the parts that we put into airplanes are designed with that situation in mind. There are two types of load stresses acting on aircraft structures; static load and dynamic load. Static strength is the ability of a part to resist a very high applied load only one time. Dynamic strength is the ability of that part to resist an applied fluctuating load of lower intensity many times.
Some of you may have flown on an airliner and noticed the wings fluctuating up and down slightly as the plane flew along. The wings have to be able to yield to the many forces that influence them. Each one of these fluctuations creates a dynamic load on the parts in the wing and it is the challenge of engineers to decide what forces the wing will see, how much the wing must yield to the changing force applications, and how many times the collection of parts in the wing will survive the dynamic load before they become “fatigued”.
In order to determine this, we “fatigue test” parts. A fatigue test is basically a long term tension test using lower loads than the part could sustain in a static environment. For fasteners, we put the bolt in the fatigue testing machine and grab on to the head and the thread of the part and pull on it with predetermined loads. We set the machine up to apply a “high load” which is usually somewhere around 37% to 45% of the ultimate load the fastener is expected to bear under static conditions. We then alternate that “high load” to a “low load” which is typically 10% of the “high load”. One application of both the high and low load is called a “cycle” and the machine applies these cycles very fast. Look at this in terms of a stressful job that might have you so fatigued – you have bad day on Monday and a somewhat stressful day on Tuesday followed by another bad day on Wednesday. If this “cycle” continues then there will likely be a failure.
We can easily illustrate fatigue by using something that is probably littering your desk right now. Grab a paperclip. You can bend it by hand. It was designed to be somewhat flexible. It expands when you slide it over the papers and then contracts to grip the papers – like the wing of the airliner we talked about earlier. You can imagine a tool that could apply enough load on the paperclip (ultimate static strength) to snap it in half but you cannot do it with your fingers. Grab one end of the paperclip and bend it out. You have just applied a dynamic load. If you keep bending the wire it will get easier and easier to bend until the wire breaks. You have just fatigued the metal to the failure point using far less load than it would have taken to cut the wire.
Questions or comments? Post them below or email me at: ed.spanknoble@heartlandfasteners.com
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